Reprocessed magnetorheological elastomers with reduced carbon footprint and their piezoresistive properties

1 Vol.:(0123456789) Scien i ic Repo s | (2022) 12:12041 | h ps://doi.o g/10.1038/s41598-022-16129-y www.na u e.com/scien i ic epo s Rep ocessed magne o heological elas ome s wi h educed ca bon oo p in and hei piezo esis i e p ope ies A. Mun eanu1, A. Ronzo a1,2, E. Ku alko a1, P. D ohsle 1, R. Moucka1,3, M. K acalik4, O. Bilek2, S. A. Mazlan5 & M. Sedlacik1,2* Despi e he as amoun o s udies based on magne o heological elas ome s (MREs), a e y limi ed numbe o in es iga ions ha e been ini ia ed on hei ep ocessing. This pape p esen s a new ype o ecyclable MRE which is composed o he moplas ic polyu e hane (TPU) and ca bonyl i on pa icles (CI). The chosen TPU can be p ocessed using injec ion moulding (IM), ollowed by se e al ep ocessing cycles while p ese ing i s p ope ies. Nume ous ypes o injec ion moulded and ep ocessed MREs ha e been p epa ed o a ious pa icle concen a ions. The e ec o he mo-mechanical deg ada ion on he ecycled MREs has been in es iga ed while simula ing he ep ocessing p ocedu e. An appa en dec ease in molecula weigh was obse ed o all he examined ma ices du ing he ep ocessing cycles. These changes a e a ibu ed o he in e molecula bonding be ween he hyd oxyl g oups on he su ace o he CI pa icles and he ma ix which is s udied in dep h. The e ec o ep ocessing and he p esence o magne ic pa icles is e alua ed ia ensile es , magne o heology and piezo esis i i y. These cha ac e iza ion echniques p o e ha he p ope ies o ou MREs a e p ese ed a an accep able le el despi e using 100% o ecycla es while in eal applica ions only up o 30% o ecycled ma e ial is gene ally used. Sma o in elligen ma e ials a e able o subs an ially change hei ma e ial p ope ies unde an ex e nal s imulus, such as s ess, elec omagne ic adia ion, pH, elec ical o magne ic ield1,2. A unique ype o such sma ma e ials a e he magne o heological elas ome s (MREs) which a e consis ed o magne ic mic opa icles embedded in an elas ome ic ma ix. These composi es a e mos ly known o hei con ollable iscoelas ic cha ac e which can be uned wi h an ex e nal magne ic ield3. As a esul , hese elas ome s can be used in a huge a ie y o applica- ions. The mos common ield o applica ions is enginee ing which in ol es ib a ion abso be s, ac ua o s and dampe s4,5. O he no able applica ions include elec omagne ic shielding, senso s and lexible elec onics6–8. La ely medical applica ions use he MREs as luid anspo a ion, a i icial muscles and cell subs a es9–14. I is clea ha he usage o he MREs cons an ly inc eases which a ises he need o eusabili y o hese elas ome s. Recycling is an impo an aspec o p oduc design om bo h en i onmen al and economical poin o iew. How- e e , no many MREs a e in es iga ed in his ega d, as mos o hem a e no able o unde go such p ocedu es. A good solu ion o hese elas ome s is he selec ion o an app op ia e ille and ma ix wi h he abili y o unde go se e al ecycling p ocesses. Ca bonyl i on (CI) is he mos common ille used in MREs due o i s supe- io magne ic p ope ies and s abili y15. In e ms o ep ocessing, exposu e o high empe a u es can lead o in e- io magne ic pa icles, howe e polyme s a e usually ep ocessed a much lowe empe a u es16. A su icien ly lexible elas ome including syn he ic silicone ma ices17, ni ile ubbe s18, and polyu e hanes19,20 ep esen he second impo an componen o he mos common MREs. The complexi y o eusing hese MREs is ela ed o he c osslinked s uc u e o he composi e’s ma ix. In con as o c osslinked ma ices, he moplas ic elas ome s (TPEs) can be mel ed and eused21,22. The moplas ic elas ome s can compe e wi h ulcanized ubbe s a oom OPEN 1Cen e o Polyme Sys ems, Uni e si y Ins i u e, Tomas Ba a Uni e si y in Zlín, T ida T. Ba i 5678, 760 01 Zlín, Czech Republic. 2Depa men o P oduc ion Enginee ing, Facul y o Technology, Tomas Ba a Uni e si y in Zlín, Va ecko a 275, 760 01 Zlín, Czech Republic. 3Polyme Cen e, Facul y o Technology, Tomas Ba a Uni e si y in Zlín, Va ecko a 275, 760 01 Zlín, Czech Republic. 4Ins i u e o Polyme Science, Johannes Keple Uni e si y Linz, Al enbe ge S aße 69, 4040 Linz, Aus ia. 5Enginee ing Ma e ials and S uc u es (eMas ) iKohza, Malaysia-Japan In e na ional Ins i u e o Technology (MJIIT), Uni e si i Teknologi Malaysia, Jalan Sul an Yahya Pe a, 54100 Kuala Lumpu , Malaysia. *email: [email p o ec ed] 2 Vol:.(1234567890) Scien i ic Repo s | (2022) 12:12041 | h ps://doi.o g/10.1038/s41598-022-16129-y www.na u e.com/scien i ic epo s/ empe a u e in e ms o hei mechanical p ope ies and di icul y o p ocessing. The main di e ence be ween TPE and ulcanized ubbe s can be ound in he s uc u e o he polyme ne wo k. Vulcanized ubbe s sha e s onge ne wo ks, a p ope y ob ained om he disul ide bonds, howe e hey a e no i o ep ocessing. The balanced p ope ies o he TPEs o igina e om hei mic os uc u e, which is gene a ed by al e na ing mu ually immiscible so and ha d elas ome ic segmen s wi h a dis inc ly di e en glass ansi ion empe a u e23. Conside ing he b oad u iliza ion o MREs, hey ha e o be highly unc ional and ide o e some di icul ies. The p ope ies o he MREs a e g ea ly a ec ed by he magne ic p ope ies o he pa icles, hei concen a ion and mic os uc u e which is ei he a well-dispe sed iso opic s a e o di ec ed in an aniso opic a angemen 24. In addi ion, he lexibili y o he ma ix is c ucial o main aining speci ic p ope ies o example he MR e ec 7. The MR e ec desc ibes he di e ence o he ma ix’s s i ness be o e and a e he applica ion o a magne ic ield25. Mos s udies use he shea s o age (G’) and he loss modulus (G″) o e alua e he MR e ec 26,27. An excess amoun o pa icles in he ma ix enhances he MR e ec howe e , highly illed MREs usually su e om he Payne e ec which could signi ican ly limi hei applica ions. The Payne e ec is obse ed as a simul aneous apid dec ease o he G’ and a local maximum o G″ abo e ce ain alues o de o ma ion. I is a common beha - iou o elas ome s embedded wi h a ille a high concen a ions and i is based on changes in he mic os uc u e o he ma e ial28. Ne e heless, his phenomenon is a he igno ed in he li e a u e dealing wi h MREs and hus needs o be u he in es iga ed. In his s udy, we use a he moplas ic polyu e hane (TPU) as a MRE which can be p ocessed and ecycled while compe ing wi h i s analogous in e ms o MR pe o mance. These p ope ies o he TPU o igina e om i s s uc u e which is composed o a wo-phase chemically bonded so and ha d segmen s29. TPU-based composi es a e used in a wide ange o applica ions30–32 howe e , hei p esence in MREs is a he limi ed20,33,34. We simula e he p ocessing condi ions and e alua e hei e ec s on he s uc u e and molecula p ope ies o he ma ices. In addi ion, we cha ac e ize he samples in e ms o mechanical and MR pe o mance using indus ial- iendly equipmen . Las ly, we p o e ha hese MREs a e sui able o piezo esis i e applica ions. Piezo esis i e sen- so s ha e been in es iga ed o decades35,36 leading o a signi ican imp o emen in a a ie y o s a e-o - he-a obo ics37. Ou MREs a e able o de ec ex e nal o ces unde de o ma ion con e ing hem in o a change o esis i i y hus allowing hem o be used as senso s. Expe imen al Ma e ials and ab ica ion o he MREs. The CI pa icles (CN g ade, i on con en > 99.5%, d50 = 6.5– 8.0μm; BASF, Ge many) we e used as he magne ic ille o he p epa a ion o MREs. Elas olan® 35A12P (BASF, Ge many; 37 Sho e A ha dness) was chosen as an app op ia e TPU ma ix. The MREs we e p epa ed by mixing he CI pa icles wi h he TPU ma ix in a ious concen a ions om 30 o 80 w %. The samples wi h he mos di e se and ep esen a i e esul s a e p esen ed he e including 30, 50 and 80w %. The mixing was pe o med using a win-sc ew coun e - o a ing mixe supplied by B abende (Duisbu g, Ge many). Each ma ix was p e- pa ed a 170°C, ollowing 1-min dosing o he mix u e and i s compounding o 4min a 50 pm. In addi ion o composi e samples, a nea ma ix was also subjec ed o he mixing p ocess in o de o in es iga e he e ec o i s deg ada ion and o compa e i s mechanical p ope ies wi h he MREs. A e cooling, he homogeneous ma i- ces we e cu in o small pieces so hey can be p ocessed wi h IM. The IM was execu ed using HAAKE MiniJe P o—Pis on IM Sys em (The mo Scien i ic, Ge many) and disk-shaped samples o 25 and 1.23mm in diame e and heigh , espec i ely, and s anda d dog-bone ensile specimens ( ype 5 acco ding o ISO 527) wi h he same hickness we e p oduced. Du ing he IM o illed samples, he op imal pa ame e s, such as he empe a u e o he cylinde , empe a u e o he mould, he injec ion p essu e/ ime and pos -p essu e/ ime, we e se up 190°C, 30°C, 450ba /7s, and 350ba /3s, espec i ely. The same pa ame e s we e se up o nea samples (TPU) wi h he excep ion o he empe a u e o cylinde which was se a 180°C. The p epa ed samples we e labelled as aw ma e ial (R0) and cha ac e ized by he me hods below. Consequen ly, he samples we e u he cu in o small pieces, compounded, injec ed unde he same condi ions as s a ed abo e, and labelled as ecycled ma e ial (R1). The whole p ocess o mixing and IM was epea ed 3 imes hus ab ica ing ecycled samples labelled R1–R3. Deg ada ion p ocess analysis. The e ec o ecycling on he pu e TPU ma ix was s udied by Fou ie - ans o m in a- ed spec oscopy (FTIR). The measu emen s we e pe o med on FTIR ins umen Nicole 6700 (Nicole , USA) supplied wi h an ATR-accesso y wi h a diamond c ys al unde labo a o y empe a u e wi h he spec a esolu ion o 2 cm–1 om 64 scans, in he ange o 4000–700 cm–1, howe e he egion be ween 2000 and 2800 cm–1 is no p esen ed due o he in insic abso p ion o he diamond c ys al. The weigh a e agemolecu- la weigh (Mw), he numbe a e age molecula weigh (Mn), and he polydispe si yindex (Ð = Mw/Mn) o he es ed samples we e de e mined om he peaks co esponding o he polyme ac ion acco ding o he abso- lu ecalib a ion me hod by he gel pe mea ion ch oma og aphy (GPC) me hod using a Wa e s HPLC sys em, equipped wi h a Wa e s model e2695 and a Wa e s model 2414 di e en ial e ac ome e (Wa e s Co po a ion, Mil o d, USA).The samples we e dissol ed in e ahyd o u ane (THF) (2–3mg mL−1), hen s abilized wi h bu yla ed hyd oxy oluen (BHT) (240mg L–1) and las ly, il e ed using a sy inge il e (0.45μm). The sepa a ion was ca ied ou using a se ies o gel-mixed bed columns (Polyme Labo a o ies L d, Sh opshi e, UK) as ollows: 1 × PLgel-Mixed-A bed column (300 × 7.5mm, 20μm), 1 × PLgel-Mixed-B bed column (300 × 7.5mm, 10μm), and 1 × PLgel-Mixed-D bed column (300 × 7.5mm, 5μm); hemobile phasecon aining he THF was s abilized wi h BHT (240mg L–1) a 40°C. The low a e o he mobile phase was se o 1.0mL min−1and he injec ion olume equal o 100 μL. All da a we e analysed using he Empowe 3 so wa e. The e lec ion spec a o he TPU samples wi h he same dimensions as used in heological analysis (see below) we e analysed using a Lo ibond RT850i (Tin ome e L d) equipped wi h xenon pulse ligh . Fo he colo ime ic e alua ion he ma ices we e aken om he o iginal pelle and hen p ocessed h ough IM. The 3 Vol.:(0123456789) Scien i ic Repo s | (2022) 12:12041 | h ps://doi.o g/10.1038/s41598-022-16129-y www.na u e.com/scien i ic epo s/ measu emen o e lec ion spec a in he ange o 360–750nm was pe o med wi h a 10‐mm ape u e size and d/8° obse a ion geome y. The a e age whi eness index (WI) was e alua ed om he ob ained spec um by a e aging 10 measu emen s a a ious posi ions on he sample acco ding o ASTME313-20. The glass ansi ion empe a u e (Tg) o he so segmen s o he ma ices was in es iga ed ia a di e en ial scanning calo ime y (DSC) me hod using he Gas Con olle GC100 (Me le Toledo, Swi ze land). The samples we e p epa ed wi h simila weigh s (≈6mg) and measu ed o a empe a u e ange o –40 o 250°C using hea ing and cooling a es o 20°C min–1 unde ni ogen a mosphe e. The Tg o he so segmen s was e alua ed using he hal -heigh echnique in he ansi ion egime. The e o o each Tg was aken as he a e age empe a u e s ep o each measu ing poin . Rheology. The heological p ope ies o he TPU elas ome s we e cha ac e ized using he o a ional heom- e e Physica MCR 502 (An on Paa , G az, Aus ia). The magne o heological measu emen s we e pe o med using a magne o-cell (Physica MRD 180/1T). The samples we e exposed o a ious ex e nal magne ic ields up o 750 kA m–1. To pe o m he deg ada ion s udy, a wa e -cooled Pel ie sys em (Physica H-PTD 200) was equipped, which enables one o each empe a u es up o 180°C o simula e he p ocessing condi ions. Fo bo h accesso ies, a pa allel-pla e geome y was used. The diame e s o pla es we e 20 and 25mm o he magne o-cell and he Pel ie , espec i ely. In addi ion, he geome y used o magne o heological measu emen was sand- blas ed and 0.5N no mal o ce was applied du ing he measu emen o he elas ic samples o elimina e any possible wall-slip a low shea a es. All measu emen s we e pe o med in he linea iscoelas ic egime (LVE) whe e he modulus is independen o s ain. The LVE was iden i ied using dynamic s ain sweeps o s ains be ween 0.001 and 10% a he equency o 1 ad s–1. A ele a ed empe a u es, he samples we e es ed unde ine a mosphe e using ni ogen. Finally, dynamic ime sweeps we e pe o med a 1 ad s–1 un il he elas ome s eached a s eady s a e o samples una ec ed by he mal deg ada ion. Fo he nea samples, u he equency sweeps we e pe o med in he 0.1–100 ads s–1 egime. Mechanical es ing. The samples o ensile es s we e p oduced di ec ly in he shape o he es specimen acco ding o he equi emen s o he s anda d es me hod using he IM p ocess. The ensile p ope ies o he TPU ma ix as well as he MREs we e in es iga ed using he ensile es ing machine M350-5 CT (Tes ome - ic Company, Lancashi e, UK) wi h a c oss-head speed o 500mm min–1. The measu emen s we e pe o med acco ding o he ASTM D638 s anda d es me hod a he oom empe a u e and subsequen ly he esul s o he ensile s eng h we e e alua ed as an a i hme ic mean using 5 samples o ype 5. The s anda d de ia ion was ob ained using i e es specimens om he s ess–s ain dependencies. The coe icien o a ia ion among all he es ed a ian s o samples was lowe han 10% and 12% o ensile s eng h and o Young’s modulus, espec i ely. Piezo esis i i y es ing. To de e mine he piezo esis i e p ope ies, an elec ome e (Kei hley 6517A, USA) coupled wi h a ensile machine (M3750-5CT, Tes ome ic, UK) and supplied wi h a load cell wi h a maxi- mum capaci y o 5 kN was used. Cylind ical samples wi h 4mm high and 10mm in diame e we e sandwiched be ween wo gilded b ass elec odes and we e g adually comp essed na owing hei mu ual dis ance by up o 25% o hei ini ial spacing (sample heigh ) a he a e o 5mm min–1. The de o ma ions we e se o 5, 10, 15, 25% o he heigh o he sample and we e kep o he pe iod o 1min du ing which conduc i i y (la e con e ed o esis i i y as a ecip ocal alue) was measu ed. The measu emen s we e pe o med a oom empe a u e. The conduc i i y (σ) was calcula ed om he measu ed cu en – ol age dependencies acco ding o he ollowing o mula: whe e is he sample hickness (dis ance o elec odes) o gi en de o ma ion, A is he nominal a ea o he sample (elec ode), I is he elec ic cu en , and U is he ol age. Resul s and discussion Deg ada ion p ocess analysis. The deg ada ion o TPU was examined by FTIR spec a o each ep o- cess cycle. In Fig.1, he ep ocessing cycles spec a o nea TPU ma ix can be obse ed. The g aph displays a peak a 3300 cm–1 which ep esen s an N–H s e ching. I is clea , ha he N–H g oup is educed du ing ep ocessing. Mo eo e , he igu e shows cha ac e is ic peaks a 2900 and 2800 cm–1 o asymme ic and sym- me ic C–H s e ching, espec i ely. These peaks d op wi h each ep ocess cycle which can be a ibu ed o he mechanical and he mal deg ada ion. In addi ion, Fig.1 displays a peak a 1700 cm–1, which co esponds o an es e linkage, which is ypical du ing he ep ocessing o such ma ices38. Finally, a oma ic amine C–N g oups we e iden i ied om he peaks be ween 1300 and 1250 cm–1 which a e mo e p onounced wi h each ecycling. The eason behind he C–N s e ching is explained in de ail la e on. I can be concluded ha FTIR spec a p o e ce ain chemical changes in he ma ix du ing ep ocessing. Gel pe mea ion ch oma og aphy. Gel pe mea ion ch oma og aphy analysis is an impo an me hod o he de e mina ion o he polyme ’s molecula weigh and molecula weigh dis ibu ion, which can be used o ack he deg ada ion p ocess du ing p ocessing. Table1 shows he changes in he chain leng h, exp essed as Mw o di e en ecycling cycles. The e ec o magne ic ille on chemical changes du ing IM p ocess, ollowed by (1) σ = I U A 4 Vol:.(1234567890) Scien i ic Repo s | (2022) 12:12041 | h ps://doi.o g/10.1038/s41598-022-16129-y www.na u e.com/scien i ic epo s/ h ee p ocesses o ecycling is obse ed using he GPC me hod. In gene al, hea ea men o nea TPU induced a educ ion in Mw caused by he mal deg ada ion o he polyme chains. Howe e , du ing he 2nd and 3 d ecycling p ocess, hese alues became almos unchanged, p obably eaching equilib ium o igina ing om he simul aneous deg ada ion and accumula ion o p ocesses in he TPU23,39. An appa en dec ease in Mw and Ð, was obse ed o he 80w % composi e in con as o he TPU sample, indica ing a chain sho ening due o he p esence o CI pa icles. In he i s ecycling cycle, highe alues o Mw and Mn we e obse ed. This phenomenon indica es he p obable bonding o he chains wi h he pa icles du ing he he mal s ess in he composi e p ocessing. As he numbe o mel agi a ions inc eases, he chains sho en and he Mw and Mn alues dec ease, indica ing ha he deg ada ion p ocesses p e ail o e he ecombina ion40. The abo emen ioned esul s e ealed ha excessi e mixing o he mel wi h CI pa icles esul ed in a mo e signi ican educ ion o he molecula weigh o he 80w % polyme by up o ~ 18%, a he han he nea TPU whe e he educ ion was only ~ 13%. Colo ime y e alua ion. An addi ional deg ada ion s udy, ele an o he indus y, was conduc ed h ough whi eness index (WI) measu emen s. In Fig.2, he alues o he WI a e demons a ed o he same ma ix du - ing di e en p ocessing s a es. Ini ially, he WI d opped by hal a e he ma ix unde wen he i s IM p ocess. This could be a ibu ed o he deg ada ion o he elas ome and he by-p oduc s o p ocessing as men ioned abo e. The ea e , he ecycled samples sha e e y simila alues o WI indica ing he same o e all p oduc . The illed ma ices could no be measu ed due o he high concen a ions o he pa icles. To conclude, despi e his me hod being mainly complemen a y, he colou assignmen o ecycled p oduc s is a quick and a p ac ical es . Mechanical es ing. Tensile s eng h es s we e conduc ed o s udy in dep h he mechanical beha iou o he nea and illed TPUs. The ensile p ope ies we e measu ed i e imes o each p ocessing cycle. The mos ep esen a i e s ess–s ain cu es o each sample o bo h nea and illed ma ices a e displayed in Fig.3. The end o he cu es o nea ma ix demons a es ha he samples b eak a high elonga ions. This indica es ha he ma ix is consis ed mos ly o elas ome ic segmen s as hese cu es a e ypical o a ubbe -like ma e ials. The Figu e1. FTIR analysis o nea TPU ma ix o each ep ocessing cycle. Table 1. Molecula weigh s and polydispe si y index ob ained om GPC measu emen s. T ea men Mn (g mol–1)Mw (g mol–1)Ð (–) R0 NEAT IM 63,600 ± 2100 131,600 ± 2100 2.07 ± 0.06 R1 NEAT IM/one imes ecycling p ocess 58,600 ± 900 119,900 ± 700 2.05 ± 0.05 R2 NEAT IM/ wo imes ecycling p ocess 57,300 ± 700 115,600 ± 600 2.02 ± 0.01 R3 NEAT IM/ h ee imes ecycling p ocess 57,700 ± 1200 115,200 ± 1300 2.00 ± 0.02 R0 80w % IM 53,100 ± 4200 98,800 ± 2200 1.87 ± 0.11 R1 80w % IM/one imes ecycling p ocess 80w % illed 55,600 ± 900 100,600 ± 200 1.81 ± 0.03 R2 80w % IM/ wo imes ecycling p ocess 80w % illed 50,800 ± 300 88,600 ± 100 1.74 ± 0.01 R3 80w % IM/ h ee imes ecycling p ocess 80w % illed 46,400 ± 900 81,500 ± 1100 1.76 ± 0.01 5 Vol.:(0123456789) Scien i ic Repo s | (2022) 12:12041 | h ps://doi.o g/10.1038/s41598-022-16129-y www.na u e.com/scien i ic epo s/ MREs on he o he hand, b eak a sho e elonga ions as he magne ic pa icles canno de o m abo e a ce ain poin o cing he ma ix o be ex ended, hus he de o ma ion ha he polyme ic pa eels is g ea e han he one ha he machine imposes. All he MREs samples ce ainly show a e y simila s ain ha dening and necking which ends up in a b eak a simila alues o s ess. The maximum elonga ion a b eak di e s o e e y ecycling p ocess o he nea TPUs, howe e he di e ences be ween he samples is negligible, only R3 possesses a li le bi lowe elonga ion. I can be clea ly concluded om he Fig.3 ha he selec ed ma ix is sui able o ep ocessing as he ma ices e ain hei mechanical p ope ies du ing each ep ocessing cycle. As shown in he g aph, he illed ma ix has a e y simila end o he R0 a e he i s wo ep ocessing cycles wi h i ually he same maximum elonga ion and sligh ly lowe ensile s ess. On he o he hand, he hi d ecycling cycle shows signi ican ly lowe bo h elonga ion a b eak and ensile s ess, which can be a ibu ed o he deg ada ion p ocess caused by ecycling and po en ial bonding be ween he ma ix and he CI pa icles du ing he ep ocessing. Addi ionally, he compa ison o he nea and illed TPUs e eals quali a i ely di e en beha iou . The i s di e ence can be seen a pos -yield beha iou whe e he un illed ma ices ha e no isible s ain so ening whe eas he illed ma e ial has dis inc i e s ess d op beyond yield poin . Fu he mo e, as can be obse ed om he Fig.3, he un illed ma e ials a e capable o sus aining eno mous elonga ion wi h pa ial e e sible de o ma ion, unlike he illed analogues, in which he p esence o he CI pa icles esul ed in h ee imes lowe elonga ion a b eak. This dispa i y is caused by he high illing o he ma ix, and he p esence o he CI pa icles which also causes he highe agili y o he illed sys em. The ensile s eng h was also g ea ly educed as shown in Fig.4, howe e , Figu e2. Whi eness Index alues o he o iginal pelle (R*), ma ices ob ained h ough IM p ocess (R0) and u he ep ocessing (R1–R3). Figu e3. Tensile s eng h p ope ies o un illed nea TPU ma ix ( ull line) and 80w % illed TPU ma ix (do line). 6 Vol:.(1234567890) Scien i ic Repo s | (2022) 12:12041 | h ps://doi.o g/10.1038/s41598-022-16129-y www.na u e.com/scien i ic epo s/ he ul ima e s eng h was less a ec ed. Fu he mo e, Fig.5 con i ms ha he illed ma ices need highe s ess o achie e he same de o ma ion esul ing in a highe Young’s modulus o he illed elas ome s. Analysing he Young’s modulus o he illed and un illed samples, i can be concluded ha he nea ma ices ha e almos he same Young’s modulus o all ecycling cycles despi e he small deg ada ion. On he con a y, o he illed samples, a ligh d op o Young’s modulus is obse ed wi h each ecycling. I is gene ally known, ha igid pa icles inc ease Young’s moduli o elas ome s41, in his case i can be also assumed ha wi h an applica ion o ex e nal magne ic ield he moduli would inc ease e en highe 42 which con ibu es o hei u u e applica ions and will be shown la e on. Rheology and glass ansi ion empe a u e. To compa e he s uc u e o he ecycled nea TPU ma i- ces, dynamic equency sweeps we e pe o med a 150°C, as shown in Fig.6. A high equencies, he ecycled ma ices show no di e ence om each o he . I seems ha he alues o G’ o hese samples will con e ge a e en highe equencies. On he o he hand, he ecycled samples show a educed G’ a lowe equencies, howe e he di e ence is no e y signi ican . This dec ease is no p opo ional o he ep ocessing cycles and he elas ic modulus di e s app oxima ely by 10% wi h each cycle. Gene ally, he G’ is educed excep o he case o R2. This beha iou is known o gene ally ollow he ensile s ess end40. The abo emen ioned end is he same as he ensile s ess in Fig.6. A possible explana ion o he d op could be associa ed wi h he deg ada ion o he TPU ma ix as men ioned abo e29. The sho e chains would disen angle as e o ma ices wi h lowe mola mass. Ano he possible explana ion would be he oxida ion o he ou e laye o he ma ices which could dec ease he ma e ial’s s eng h du ing p ocessing43. Ne e heless, he ou come o he changes caused by he hi d ecycling p ocess in e ms o G’ seems o be mino om a p ac ical poin o iew. Figu e4. Tensile s eng h o he nea and 80w % illed TPU samples. Figu e5. Young’s modulus o he nea and 80w % illed TPU ma ices. 7 Vol.:(0123456789) Scien i ic Repo s | (2022) 12:12041 | h ps://doi.o g/10.1038/s41598-022-16129-y www.na u e.com/scien i ic epo s/ In o de o simula e he sample du ing he ecycling p ocess, dynamic ime sweep es s (angula equency 1 ad s–1 and s ain wi hin LVE) we e pe o med a 170°C o speci ic samples. In Fig.7, he nea TPU ma ix is compa ed wi h hei illed analogue. The nea ma ix shows a ypical polyme ic beha iou wi h he G’ dec easing wi h ime un il he chains a e elaxed and G’ becomes independen o ime. The abo emen ioned beha iou could hinde a deg ada ion p ocess; howe e , i canno be e alua ed o his ma ix wi h his me hod. Mo eo e , based on he las obse a ion, i can be con i med ha a po en ial cu ing o c osslinking p ocess (i any) is insigni ican due o he dec easing end o G’. The 30w % illed ma ix, on he o he hand, shows a simila d op o G’ o he i s 50min be o e a no ewo hy inc ease o G’ is obse ed. This can be a ibu ed o he co alen bonding which occu s be ween he ba e CI pa icles and he ma ix44. These pa icles a e co e ed wi h hyd oxyl g oups a hei su ace which can eac wi h he end-g oups o he TPU chains45,46. The abo emen ioned s a emen is u he suppo ed by Fig.7 b) in which wo MREs wi h di e en concen a ions a e compa ed. I can be clea ly seen ha o he 80w % illed ma ix, he inc ease o he G’ s a s immedia ely and is o de s o magni ude highe han he 30w % MRE. In addi ion, he beha iou o he 80w % is e y impo an in he p esen s udy as his inc ease akes place du ing he same ime-window as he ab ica ions and ecycling p ocess. On he o he hand, o 30w % his inc ease can be conside ed insigni ican as i can only be no iced a e 50min, which co esponds o mo e han en ep ocessing cycles. I is wo h men ioning ha a simila es was pe o med wi h he sample exposed o O2 by emo ing he hood which p o ided a N2 based en i onmen . A sha p inc emen o G’ was obse ed o all samples due o eac ions wi h oxygen. The la e beha iou can lead o in e io magne ic pa icles o ma ion (i on oxida ion), bu such by- p oduc s a e o med a much highe empe a u es16. In Fig.7 hough, all ma ices a e unde do man condi ions. Figu e6. Dynamic equency sweeps o he nea TPU samples a 150°C. Figu e7. Dynamic ime sweeps o (a) R0 PURE open ci cles and R0 30w % hal open ci cles and (b) R0 30w .% hal open ci cles and R0 80w .% illed ci cles. 8 Vol:.(1234567890) Scien i ic Repo s | (2022) 12:12041 | h ps://doi.o g/10.1038/s41598-022-16129-y www.na u e.com/scien i ic epo s/ The Tg o he so segmen s was e alua ed using he DSC me hod as i is a c i ical pa ame e o magne o heology34. The Tg alues o he TPU 80w % MREs a e shown in Table2. An appa en inc ease o Tg is obse ed a e he i s ep ocessing. The e a e wo main easons o he Tg o inc ease. The i s indica es an incensemen in Mw, howe e , conside ing he GPC measu emen s ha is no he case. The second and mos p obable eason is he limi ed mobili y o he chains o hei s i ening. I has been epo ed ha TPU ma ices a e p one o in e molecula bonding du ing p ocessing47. I has been obse ed o pa icles o he han CI o bond wi h his ma ix48. As a esul , he mobili y o he chains is educed leading o highe alues o Tg. As shown in Fig.8b, he possible bonding seems o be in ense only du ing he i s minu es o p ocessing. This is a ibu ed o he a e ha G’ inc eases which is cons an ly d opping and is indica ed by he conca e cu a u e. This is in ag eemen wi h he Tg alues eaching an equilib ium a e he second ep ocessing which co esponds o a leas 10min o p ocessing no coun ing he ime o cool down. Las ly, he alues o he Tg ha e a signi ican in luence on he MR e ec . Fo a di e en TPU wi h a simila Tg bu illed wi h he same concen a ion o CI pa icles, a 9°C inc ease o he Tg co esponded o a ~ 70% dec ease o he MR e ec 34. Fo he ecycled samples, he bond- ing be ween he pa icles and he ma ix supp esses he MR pe o mance, hus only injec ion moulded ma ices we e s udied. To conclude, he abo emen ioned bonding educes he e ec s o deg ada ion on he mechanical p ope ies, and he MR e ec is supp essed o he ecycled ma ices. Howe e , in he eal applica ions, only one hi d o he ma ix will be ecycled hus he MR pe o mance o he injec ion moulded samples will no be changed d ama ically. The magne ic p ope ies o he TPU ma ices illed wi h CI pa icles we e e alua ed a oom empe a u e using dynamic s ain sweeps unde a ious homogeneous magne ic ields h ough he MR e ec which is de ined in he ollowing o mula: whe e he on- and o -s a es a e e e ed o as he p esence and absence o he magne ic ield, espec i ely. Gene ally, samples wi h pa icle concen a ions below 50w % do no show any signi ican MR e ec which was obse ed in he pas o a e y simila sys em33. On he o he hand, he 80w % ma ices a e oo immobile due o he bonds wi h he ma ix and he e o e an insigni ican MR e ec was obse ed. Fo ha eason, a new ma ix illed wi h 50w % was p epa ed and he MR pe o mance was s udied h ough dynamic s ain sweeps. Figu e8a shows an inc ease in G’ when he samples a e exposed o ex e nal magne ic ields. The inc ease o G’ is a ibu ed o he magne ic pa icles a emp ing o align hemsel es and o m chain-like s uc u es pa allel o he magne ic ield. In addi ion, his end is p esen mainly du ing he LVE whe e G’ is independen o s ain, which can be dis inguished by he g een solid line in Fig.8a. Abo e he c i ical s ain, he cu en s uc u e collapses indica ed by he sudden d op o he G’. The da a in he LVE seems sca e ed especially a lowe s ains which is a ibu ed o he high illing and s i ness o he ma ix. Ano he di icul y is connec ed wi h he alues o o que a lowe s ains which a e only a ew imes highe han he esolu ion o he ins umen . The la e is commonly encoun e ed o highly illed ma ices49. As a esul o he sligh ly sca e ed da a, he a e age o hese poin s was conside ed o he e alua ion o he MR e ec . Unlike he esponsi e LVE, he magne ic ield does no a ec he modulus soon a e he c i ical s ain is eached. Thus, he MR e ec o a ious ields was ob ained only om (2) MR e ec = G′ on − G′ o G′ o Table 2. The Tg alues o he so segmen s a di e en ep ocessing cycles o he 80w % MRE. R0 R1 R2 R3 Tg (± 0.3°C) –17.4 –13.1 –14.5 –14.4 Figu e8. (a) Dynamic s ain sweeps o samples con aining 50w % CI pa icles a di e en magne ic ields ( illed symbols; ci cles 0kA m–1; iangles 150kA m–1; s a s 450kA m–1; diamonds 750kA m–1); nea TPU ma ix (un illed) and (b) MR e ec o he same 50w % ma ix. 9 Vol.:(0123456789) Scien i ic Repo s | (2022) 12:12041 | h ps://doi.o g/10.1038/s41598-022-16129-y www.na u e.com/scien i ic epo s/ he LVE. A linea incensemen o he MR e ec is demons a ed in Fig.8b. These esul s can be compa ed wi h simila elas ome s, hus hese p ocessed TPU a e able o compe e and su pass o he TPU-based MREs on he MR e ec poin o iew33,34. Las ly, i is e y impo an o add ess he Payne e ec as i could become a po en ial p oblem o u u e appli- ca ions. Despi e being obse ed in MREs and highly illed ma ices as a sudden dec ease in G’, simila o he one in Fig.8a, his e ec is comple ely igno ed in mos o he s udies ega ding MREs. Fo he MREs speci ically, i expe iences mo e e ec s such as a s eady inc emen o G’ wi h each consequen ial measu emen making i wo h in es iga ing28. Howe e , he ocus o his s udy is o only e alua e he p esence o he Payne e ec . In Fig.8a, he nea TPU ma ix is also illus a ed wi h a simila o e all beha iou as he illed samples. The wo main di e ences include he alues o G’ in he LVE which a e highe o he illed pa icles and he ex ended LVE egime o he nea ma ix. Bo h obse a ions a e common o highly illed ma ices50. Since he pu e ma ix also shows a d op in he neighbou hood o he illed pa icles i is sa e o conclude ha any po en ial Payne e ec could exis only in he limi ed s ain egime a ound 0.02 and 0.2% whe e he LVE is e mina ed o he illed and nea ma ices, espec i ely. Howe e , as men ioned be o e, ma ices wi h high ille concen a ions ha e hei LVE sho en50. As a esul , he d op in G’ is mainly caused by he collapse o he polyme ic s uc u e, a he han he collapse o he ille ’s ne wo k which should be mo e obus ega dless due o he pa icle–ma ix bonding o he ma ix as men ioned abo e. Piezo esis i i y es ing. In gene al, when a conduc i e composi e is exposed o comp ession, i s con- duc i i y is inc eased ( esis i i y is dec eased) as a esul o he piezo esis i e e ec which is widely exploi ed51. As can be obse ed in Fig.9, he dependence o he esis i i y, used equen ly as a quan i y desc ibing he pe o mance o piezo esis i e senso s, a a ious comp ession a es o bo h nea and illed ma ices a di e en concen a ion and ep ocessing cycles is p esen ed. The esis i i y is dec eased a highe comp ession s ains. Fo he nea TPU ma ix, once a comp ession o 5% is achie ed, a s eep dec ease o esis i i y is appa en which is ollowed by a sa u a ion as he samples unde wen u he de o ma ion. Mo eo e , he ma ices illed wi h he CI pa icles showed a no able inc ease in conduc i i y. The la e ema k has been obse ed be o e in aniso opic MREs 13. In he same s udy i was sugges ed ha he dec ease o esis i i y was achie ed wi h a di e en mecha- nism known as he a iable- ange hopping mechanism. O e all, piezo esis i i y inc eases wi h concen a ion and comp ession, ne e heless i sligh ly dec eases wi h ecycling. A possible cause could be he c ea ed bonds be ween i on and ma ix du ing ep ocessing. Finally, Figu e9. Resis i i y dependences on he ela i e comp ession de o ma ion o he nea TPUs (open symbols), 30w % illed TPUs (hal up open symbols) and 80w % illed TPUs (solid symbols) o (a) IM/samples ( ed), (b) IM/one ime ecycling p ocess (blue), (c) IM/ wo imes ecycling p ocess (g een), (d) IM/ h ee imes ecycling p ocess (pu ple).